The present invention relates to a method and apparatus for the making of thin-film electrode patterns, and more particularly, to the use of electrochemical reactions to pattern thin-film electrodes on a substrate carrying a film of electrode material.
Thin-film electrodes are used in various types of electronic devices including liquid crystal displays. In these types of devices, a layer of liquid crystal material is confined between a pair of spaced conductive members, at least one of which carries electrode material in the form of a pattern of thin-film electrodes. The electrodes are typically formed from a transparent conductive material, such as indium oxide or indium tin oxide. Various electrode patterns are used, including patterns that define numeric characters, alpha-numeric characters, and arrays of picture elements. The application of a voltage potential across selected areas of the electrode segments locally alters the orientation of the liquid crystal molecules to correspondingly alter the optical characteristics of the liquid crystal material and to achieve the desired optical display effect.
Various methods have been developed to deposit and selectively etch thin-films to define a desired electrode pattern. Using high vacuum deposition techniques, a film of conductive material is typically applied, uniformly across the surface of a substrate. The film can be deposited by, for example, thermal or electron-beam vacuum deposition, sputtering, or vapor plating. After deposition of the conductive film, a layer of photoresist material is applied and selectively exposed to ultraviolet or other appropriate radiation directed through an apertured mask that carries the desired electrode pattern. The resist polymerizes in the radiation exposed areas. After exposure, the unpolymerized resist, which corresponds to the unexposed areas, is removed using developing and/or washing solutions. The remaining polymerized resist can be subjected to further processing steps including chemical fixing, hardening, and heat treatment steps. The areas of the conductive film not covered by the fixed resist are etched with an acid to remove the unprotected conductive material. Subsequently, the remaining resist is removed to provide a substrate bearing the desired conductive electrode pattern. A variation of the photolithographic technique involves direct screen printing of the resist pattern onto the conductive film, this latter variation being used where precise dimensional tolerances are not required. Other pattern formation techniques include the direct deposition of the patterned electrode onto the substrate by deposition of the conductive material vapor through an appropriately apertured mask or spraying a chemical compound through a stencil with the chemicals reacting to form the appropriate electrode pattern.
The known processes for forming conductive electrode patterns have various advantages and disadvantages. In general, the photolithographic methods provide precisely formed electrodes. They require, however, many processing steps and are oftentimes associated with low yields, owing to delaminations and the adverse affects of such steps on conductivity, optical or electro-optic properties. On the other hand, the techniques which print the resist pattern or directly deposit the conductive material by plating through a mask or stencil generally do not produce precisely spaced, high resolution, high-acutance electrode patterns.
Liquid crystal displays have been used in increasing numbers as efforts have been made to reduce the manufacturing costs of these devices. In one approach, and disclosed in U.S. Pat. No. 4,228,574 to Culley et al, two elongated plastic preform strips are each prepared with electrodes and alignment layers. Adhesives seals are deposited onto one of the two preform strips to define the perimeter of each to-be-formed cell. Discrete quantities of liquid crystal material are deposited onto one of the strips in general registration with the adhesive seals and the two strips are joined together to form an elongated strip assembly of serially arranged cells. While the use of flexible plastic strips is conducive to automated production techniques, the electrode patterns must nonetheless be applied in a step-wise, non-continuous manner that limits the ultimate production efficiency of an automated manufacturing technique. Accordingly a need exists by which electrode patterns can be efficiently applied to an electrode bearing substrate in a manner conducive to automated production techniques.